Magellan和Garmin的几个问题
本文中的疑惑和观点仅仅表示个人的一点想法。
前面有个老外在房间里作的比较测试，翻翻前面的帖子http://www.gearer.net/viewthread.php?tid=4825&highlight=60csx，测试中Garmin60Csx能够锁定的卫星数量颇为令人惊讶，最多的有10颗之多。根据个人经验，用老机器在海上曾经有过锁定10颗卫星的情况。房间里仅仅通过窗口锁定这么多卫星，个人能够理解的解释（猜测）就是通过提高接收机灵敏度，锁定通过窗口传输进来的卫星星历数据，其中应该包括由于多径效应产生的信号，也就是说应该有些可见性不好的卫星信号可能通过房间外面的建筑物或者其他甚么东西反射进房间内，老机器的灵敏度不高可能收不到，但是通过接收灵敏度的提高还是可以捕捉到这些信号。
另外关于静态漂移产生的原因，一直想不明白。曾经记得一个老外写过一个帖子，大意是他认为静态漂移是由SA产生，不过米国已经宣布取消SA策略，个人觉得这并不是一个合理的解释。也有老外说Magellan的漂移比Garmin小是因为前者在算法里面定位数据取了平均，这个解释合理，因为该测试的原文有这么一段“The most interesting thing is that if we do a straight average of all the data from each receiver, we do get exactly the same coords...”似乎前面的帖子没翻这句。另外，该问作者也给出了这种算法的问题“It's worth mentioning that the Explorist behaviour may seem 'clearly better', but it's misleading for certain tasks. It seems pretty clear that it's performing an averaging over time. For display on-screen of your position, that's fine. But it's misleading in the NMEA datastream to report "This is your position at time T" when the reality is that it's your positioned over the recent time period T1 through T2. That makes the raw data less useful for further data analysis since it's already been processed to an unknown and variable degree. So if you were in the business of using the NMEA datastream for hyper-accurate positioning, the Magellan's behaviour is probably not what you want. If you were actually moving, or just using this to drive a moving map program, this distinction wouldn't matter.”
关于这个问题一个德国人也作了一些测试，引一段“It happened several times that I forgot to turn off the 60CSx when I entered a building (e.g. a restaurant) during a day trip. I have set my tracklog recording to "most often". Together with the jumpy reception in building, this results in a new track point recorded every few seconds. Staying two hours in a restaurant can cost you several thousand points of the tracklog memory. Once I lost 8000 points in about three hours. You loose much less points if you set the recording to "more often" or "normal", but the problem is still there. After 4 weeks of using the new model I got used to turn it of inside a building to save tracklog memory.
The situation is much better if you have at least a fair reception. The 60CSx recorded 121 points (see the picture of the tracklog below) after sitting in my office near the window for 1 hour and 24 minutes with recording mode set to "most often". It "traveled" 493 meters during that period, which is acceptable considering the poor reception.”
这个结果，比版上帖子里的60CSx的结果好的多，跟探险家的结果相当。
最后，还有一个老外认为晚上接收机的精度最好，其次是下午。因此结合上面的结果，想到测试环境也许对测试结果有比较大的影响。手里只有一台老机器，没法作这些测试，但是觉得对比测试应该能够具有可再现性，希望哪位能够拿到这些机器的老大考虑做个验证性测试。
最后对这个静态漂移指标再说几句。版上的那个帖子给出了位置漂移，但是没有说高度通道的事。不清楚GPS不跟气压高度表组合时，静态高度漂移有多少。再就是最后的定位精度应该考虑高度吧，即3维误差是多少是不是更有说服力些。
最后，一个最大的疑惑就是，暂不考虑测试因素，这个静态漂移指标的意义在哪里？尤其是对于户外活动的意义？由于搞不清楚静态漂移产生的原因，所以不知道它会不会对动态精度构成影响。
最后，俺选择GPS的时候还比较关注另外一个指标，不知道各位老大怎么认为。我觉得机器的温度适应性也要考虑。Garmin的指标是 －15度到 70度，Magellan的是 －10度到 60度，我想这可能也是为啥Garmin大一些贵一些的一个原因吧。 |
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发表于 2007-5-20 21:15
发表于 2007-5-20 21:21